Method of pressing sheet lumber



Feb. 20, 1951 w, c, 055 2,542,025

METHOD OF PRESSING SHEET LUMBER Filed July 1, 1946 20 E M- /2 \f 20/ l IT\- i i 30 i I m 1| mm m llllllllll 20 f a: Axe-.4: "5:2: 52:04.

IN VEN TOR.

W RTH C. G055 Patented Feb. 20, 1951 Worth 0. Goss, Seattle, Wash,assignor, by direct and mesne assignments, to Edward S. HellerApplication July 1,- 1946,:Serial No. 680,839

' This invention relates to the manufacture of products which may belike or of the general character of the sheet lumber products describedin my copending application filed December 22, 1945 under Serial No.637,107 now U. S. Patent No. 2,485,587 and in an application filed underSerial No. 680,838, executed June 24, 1946, and entitled "Sheet Lumberand Its Method of Manufacture," which now has been abandoned in view ofan application filed under Serial No. ,759 as a continuation in partthereof and in which applications, I have described a method of makingsheets of lumber from a mixture of fiberized ligno-cellulosic materialsand a hydroplastic bonding agent; the mixed fiberized mat'erlal beingsifted to form a homogenous blanket of material which is pressed, whilewet, into a solid sheet between press platens and the fibers andbondingagent thereby coalesced together by theaction of the heat and pressure.In' the making of a lumber sheet in accordance with the teaching of thefirst application above mentioned, the technique of manufacturecontemplates that a platen pressure of approximately 50 pounds persquare inch and a platen temperature of about 350 F. be used in formingthe sheet and that the sheet be retained in the press under this heatand pressure until completely dry.

'jlhe teaching in the second application above mentioned is that thepressed sheets may be removed from the press after their moisturecontent has been reduced by the heat and pressure team or less and thedrying completed later; forj'example, completed in a kilndesigned forthis purpose.

' In the manufacture of sheet lumber by the method of either of theabove applications, it is required that the press be vented for theready escape from within the sheet of steam and gases that are incidentto the application of heat and pressure to the wet mixture. Furthermore,since it is desirable for various reasons, that one surface of thefinished sheet be perfectly smooth, this necessitates the use of aperfectly smooth surfaced press platen for forming tithis particularsurface. The other platen, then, must necessarily bevente d, and thiscan be accomplished by placin a fine mesh wire screen between the layerof sheet forming material and the platen, as is quite well known in theart. Also, I have found it"desirable and advantageous that this platen,at the steam escape surface, be formed with slots or channels in thatsurface against which the screen is disposed, thus to facilitate thequick escape of dispelled gases, steam outside of the press.

' The advantages in and desirability for the removal of the pressedsheets from the press while wet has been fully explained in the aboveseedud-mentioned application. Also it has been explained therein thatthe pressing technique deand water to the 6 Claims, (01. 18-475) UNITEDSTATES PATENT dome: f;

scribed causes the lumber sheets to be formedwith extremely dense,welded surface layers and an. intermediate body portion that is oflesser density; this latter being due to the removal of the sheet fromthe press while still wet. The thickness of the welded surface portionswill dee pend, among other things, on the platen temper-' ature used andthe length of time of pressing.

It has been found that if the usual presentday types of presses are usedfor the production of lumber sheets that can be removed from the pressplatens while still wet, the dense surface layers that are formed willbe of different thickness; that is, the welded surface layer of thesheet which is adjacent the smooth surfaced bottom platen will be ofgreater thickness than that Welded surface layer which is adjacent thescreen' and upper platen; this being due to the fact that when theplatens of such presses are heated to equal extent, the upwardlyescaping steam and gases will maintain a higher percentage of mois-"-ture' at this surface and thus reduce the effective temperature of theplaten accordingly.

'The principal disadvantages of having the sheet formed with surfacelayers or flanges of unequal thickness are: first, that the sheet will,upon removal and drying, warp or curve toward the side of greaterthickness; and, second, the sheet will have unequal bending strength inopposite directions, which is not desirable.

In view of the above explanatory matter and- 1 the noted disadvantageresulting from the forming of welded layers of unequal thickness, it hasbeen the principal object of this invention to provide a novel methodand means for manufacturing that particular kind of sheet lumber whichhas extremely dense, welded surface layers and In Carrying out yinvention, and to attaiiif. the objects above mentioned, selectedligno-cellulosic materials such'as wood chips, o other wood wastematerials, are steamed under atmospheric pressure steam and then, Whilehot and wet, are fed through an attrition type grinder together withabout 15% of a suitable plastic binder, which preferably would be thatbinder or bonding agent described in my application filedDecember 22,

1945, under Serial No. 637,107, now U. S. Patent No. 2,485,587, butwhich might be any other suit-' the sheet the necessary strength toresist internal disruptive forces if. removed from the pressbefore beingcompletely dry.

The main requirement for Whatever binder may be used, is that it havethe necessary adhesive power to maintain the sheet. intactas pressed,when it is removed from the press platens in a partially driedcondition.

If addition of the plastic or bonding agent'to" the wet fibers iscarried on in conjunction with the fiberizing of the wood in anattrition type grinder, the agent will be evenly and effectively appliedto the individual. fibers, and thus the adhesive property of the plasticwill. be made most effective... It will be mentioned, however, thatstheselected plasticor agent might, as. an alternative, be. mixed with. the.fiberized material after the fiberizing operation by placing therequired-proportionate amounts of fibers and agent iniany of the wellknown types of rotary mixers. The. exact manner of mixing the fibers'andhydropl'astiebonding agent is not critical except .to the extent. thatit be .efiectively and completely done; In the making of .sheet lumber.in:accordance with the present method, the fibrous. mixture as.delivered'into the press for consolidation into' sheets, would containfrom 40% to 150% moisture, and this, in the two stage pressingoperation.

used, isreduced to around 40% or less before the sheet is removed fromthe press, as presently explained.

- 'After. the fibersand bonding agent have been. properly mixed, themixture is distributed, preferably by sifting it throughiascreen, onto amoving-belt to form a blanket thereon of that .desige nated thicknessrequired for the formation of a. sheet therefrom of. a. given densityand final thickness by the operation of heat and pressure. Then .thebelt is advanced to. locate the blanket of. fibers between. the heatedupper and lower platens of a press, such for example as that typeemployedin pressing veneer sheets, but modified in accordance with theteaching of :the present invention with respect to platen temperatures,as presently explained.

The upper platen. should be provided with grooves or channels across theface thereof. for the quickand effective venting ofv steam and a wirescreen. of from... 14 to meshplaced between this. platen surface and thefiber blanket. The: lower platen surface, or surface of whatever elementis used to form. the lower surface of. the. sheet,- should be perfectlysmooth and not equipped for steam venting.

, As. distinguished. from the usualprocedure. of heating upper and lowerplatensto thesame. temperatures, the present methodand'its advantagesare. predicated. uponthe heating. of the upper. and.

lower platens to difi'erent. temperatures;v for example, inmaking. thesheet lumber of the second mentioned pending. application, I heat theupper.

steam vented platen to a temperature of. about.

450 F., and heat the lower, smooth platento about 350 5.. Thedifferential in temperatures of the platens is required in order. toeffect the formation of hard surface portions of equal thick-. mess onboth sides. of .the sheet. The. difference in platen temperatures wouldbe. determined in accordance with materials used and pressures.employed; and establishedby trialjor. test. The gistof the inventionresides in utilizing. that differentialin temperatures. between. upperand lower platens that will, for. the certain materials used andtemperatures andv pressures. employed, result. in. the formation. of.welded; surface layers 4 I I of the same thickness. This thickness couldbe controlled by the lengthof time the sheet is maintained under heatand pressure.

For the making of a sheet of lumber that is especially desirable forbuilding purposes, the press-platens, heated to the temperature as abovegiven, are closed against the fiber blanket at a pressure of from fourto six hundred pounds per square inch, and this pressure is maintainedfor a period. of. not exceeding ten seconds. Then the platens areretracted. The pressing operation is then resumed in the same or in adifferent press under the same conditions of heat, but at about fiftypounds persquare inch, and, pressing at the. temperature describediscontinueduntil. the; over-all moisture content of the sheethasgbeen.reduced to about 4.9% or less, figured on the: basis of the dry board;Then the sheet is removed. from thepress for its final drying;vThismaybe completedin a heated kiln designed forthe purpose. Itis tobeunderstood that the action of heat and pressure applied as abovestated, sets the; bonding agent and causes the fibers to beheld togetherwith the necessary adhesiveness to 1'83 sist any disruptive. action ofentrapped steam andgases upon release of the platen pressure}. The-finaldrying gives additional strength to the. sheet andv leaves the lessdensely welded portioncomparatively porous. The welded surface per.-

tions, being of equal thickness,., insure against.

warping. and give great strength forlow weight to the sheet.

It is further to be. explained that. since it. is usually. desirablethat the finished sheets. be of a. definite. and predeterminedthickness,the. pressis equipped with limitingstopswhich are eifecw tive in thesecond stage of pressing and which definitely limitthe extent to whichthe. platens may closetogether regardless. of. the. length of. thepressing period. After completion of the first stage of pressing, at thehigh pressure of" from- 200 to 600 pounds, with platen. temperatures atapproximately 450 F. and 350 F.,. re-

spectively, the press is opened, the stopsinserted.

and. the second stage of pressing. is started." Gradually, the platenswill close and the sheetcompressed until. their approach toward eachother is limited by the stops. This should come about one minute beforethe end of this pressing interval. The heat of the platens, however,continues to drive out the moisture from the sheet, which is. not.removed until its moisture content. has. been reduced to. the desiredpercentage of 40%. or below. In this. process, all moisture .percentagesarefigured ona dry weight basis. v

Should itbe found desirable for any special use, to. provide sheets.with welded surf-ace flanges. of. unequal thicknesses, this. can.

plished. by use of platen temperatures so. regur;

readily accom lated asto effect. the desired. results, or by cut-jtingoff the. heat. fromone. platenprior to, the other to retard ordiscontinue the welding. action at that side priorto the other.

A press suitable for carrying out the present method has beendiagrammatically shown in the. accompanyingdrawings, wherein-.-

Fig. l is a .view showing press platensasinie tially applied to.a..layerof fibers for the sheet pressing operation.

Fig. 2 showsthe sheet as formed by the. initial pressing operation athigh pressure.

Fig. 3 is a view of thepress platens aslimited in theirclosing. actionduring. the second. stage of: the sheet pressing. operation.

Fig. 4 is an enlarged cross sectional view of a part of a finished sheetindicating the difference in density of its surface and centralportions.

Referring more in detail to the drawingslfl designates what may be thelower platen of a press, and H designates the upper platen. i2 indicatesany suitable mechanism whereby the platens are moved toward or from eachother. The lower platen has a smooth top surface l and the upper platenhas a number of narrow mouthed channels [3 formed at regular intervalsof spacing in its bottom surface for steam escape.

In Fig. 1, l5 designates a blanket of fibers as advanced onto the pressbetween the platens for consolidation, and I6 designates a wire meshscreen placed between this upper platen and the fiber blanket.

and 20' designate stops in the form of square bosses that are appliedbetween the platens during the second stage of pressing to determine thefinal thickness of the sheet.

In Fig. 4 I have shown a part of a finished sheet in enlarged crosssection. In this View, the hard dense surface layers are represented bythose parts within the brackets 26 and 26 and the less dense portion isthat within the bracket 21.

It will be understood that there is no pronounced line of demarcationbetween these layer portions or flanges and the central portion, butthat they gradually merge one into the other, and that the relativethickness of the parts 26 and 21 is determined by pressures,temperatures and kind of material used.

Heating of the platens may be eifected by gas, electricity or otherwise.In Fig. 1, I have indicated the platens as being equipped withelectrical resistance heating elements 30 and these would be so madethat each would produce the heat required in each platen.

Having thus described my invention, what I claim as new therein anddesire to secure by Letters Patent is 1. The method of manufacturingsheet lumber comprising reducing wet ligno-cellulosic material to afiberized condition, mixing therewith a plastic bonding agent having theproperty of providing adhesive strength while wet, laying down ahomogeneous matted pad of said mixture and pressing the pad into aunified, non-expandable sheet between press platens, one of which has asmooth surface and is heated approximately to 350 F. and the other ofwhich has a steam vented surface engaging the pad and is heated toapproximately 450 F.; the pressing operation comprising a first stage ofpressure applied for a relatively short time at from 200 to 600 poundsper square inch to cause the sheet to be formed with densely weldedsurface flanges, and a second stage of pressing at a materially reducedpressure, and the sheet being removed from the press While that portionbetween said flanges is still wet.

2. The method as recited in claim 1 wherein the second stage of thepressing operation is carried on at 50 pounds per square inch or less,and the sheet, after being removed from the press is subjected to finaldrying in a kiln without consolidating pressure being applied.

3. The method of making sheet lumber comprisin the steps of forming amatted, homogeneous blanket of wet ligno-cellulosic fibers andcompressing the said blanket into a unified, nonexpandable sheet betweenthe platens of a press 6 wherein one platen is formed with a smoothpressing surface and is heated to approximately 350 F. and the otherplaten has its pressing surface vented for the escapement from that sideof the sheet of steam created in the pressing operation, and is heatedto approximately 450 F. to cause the resultant welding together offibers in dense flanges to progress inwardly from the opposite surfacesat equal rate and removing the compressed blanket from the press whilethere is a medial portion between the flanges containing a substantialpercentage of moisture.

4. The method of making sheet lumber comprising forming a blanket ofligno-cellulosic fibers having from 40% to moisture content andcompressing the blanket into a rigid sheet between the platens of apress wherein one platen has a smooth pressing surface in direct contactwith the blanket, and the other platen is vented at its surface forescapement of steam from that side of the sheet, the smooth surfacedplaten being heated to approximately 350 F. and the steam vented platenbeing heated to about 450 B; said blanket being maintained under saidheat and pressure until hard, dense surface layers have been formed onthe sheet and the fibers have been coalesced throughout the sheet, andremoved from the press while the medial portion of the sheet stillcontains a substantial percentage of moisture.

5. The method of making a sheet of lumber, with one smooth surface andwith dense, welded surface layers and a less dense body between the saidsurface layers which comprises forming a mixture of moistligno-cellulosic fiber and a bonding agent into a matted pad andcompressing the pad into unity between heated press platens, one ofwhich platens has a smooth pressing surface and is heated toapproximately 350 F., and the other of which platens has its surfacevented for steam escapement and is heated to approximately 450 F., andremoving the sheet from between the platens after the fibers thereofhave been coalesced sufficiently throughout the sheet to prevent itsexpansion and while the sheet still contains moisture in the medialportion thereof.

6. The method of making a sheet of lumber with one smooth surface anddensely welded surface layers and a less dense body between said layers;comprising forming a, moist mixture of ligno-cellulosic fibers andbonding agent into a matted pad, compressing the pad into a rigid sheetbetween heated press platens, one of which platens is smooth and theother is steam vented over its pressing surface; the smooth platen beingheated to a degree that will effect a quick welding of the fibersadjacent thereto to form a dense surface layer, and the other platenbeing heated to a substantially higher degree to counteract the coolingeffect of escaping steam, and to form a dense surface layer of likethickness on that side of the sheet.

WORTH C. GOSS.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,595,414 Mohler Aug. 10, 19261,844,861 Mason Feb. 9, 1932 2,080,078 Mason May 11, 1937

